fix: etcd watch restart when receive invalid revision

[Revolyssup]

Description

 [error] 56#56: *223892756 lua user thread aborted: runtime error: /usr/local/apisix/apisix/core/config_etcd.lua:260: attempt to compare number with nil
stack traceback:
coroutine 0:
    /usr/local/apisix/apisix/core/config_etcd.lua: in function </usr/local/apisix/apisix/core/config_etcd.lua:130>
coroutine 1:
    [C]: in function 'ngx_thread_wait'
    /usr/local/apisix/apisix/core/config_etcd.lua:285: in function </usr/local/apisix/apisix/core/config_etcd.lua:268>, context: ngx.timer
 [error] 56#56: *223892756 lua entry thread aborted: runtime error: /usr/local/apisix/apisix/core/config_etcd.lua:287: attempt to concatenate local 'err' (a boolean value)
stack traceback:
coroutine 0:
    /usr/local/apisix/apisix/core/config_etcd.lua: in function </usr/local/apisix/apisix/core/config_etcd.lua:268>, context: ngx.timer

When the revision contained in the response returned by etcd watch API is empty, it will cause the watch thread to abort. Since the thread exits abnormally, the error object return from ngx.thread.wait is either nil or a boolean value (nil can be reproduced, but boolean cannot, I don't know why yet), causing subsequent log printing to abort too and making it impossible to restart the watch thread.

Fixes # (issue)

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[membphis]

code style: bad indentation

[membphis]

log.error("run_watch or check_worker thread terminates failed",
                " restart those threads, error: ", inspect(err))

[Revolyssup]

done